As the electronics and communication techniques have been substantially developed, a wireless speech calling service and a wireless Internet service using a wireless communication network have been provided. Recently, as the main mobile communication services have been switched from the voice service with the constant data rate to the packet service with variable data rates, the OFDMA scheme for easy interference hindrance has been used other than the CDMA scheme for maintaining predetermined interference through interference averaging.
The OFDMA based mobile communication system (i.e., OFDMA system) has used the frequency division duplex method (FDD) and the time division duplex (TDD) so as to transmit and receive signals between the subscriber station and the base station.
In the TDD method of the OFDMA system, a downlink and an uplink coexist in the frame structure, and as shown in FIG. 1, a Tx/Rx transition gap (TTG) and an Rx/Tx transition gap (RTG), that is, guard time for identifying forward and reverse transmission time are inserted between a downlink and an uplink. In order to synchronize the downlink time in the OFDMA system, the subscriber station measures the preamble signal of the downlink provided by the base station and corrects downlink time synchronization.
In the OFDMA TDD based uplink, the subscriber station is to be synchronized with the base station. The subscribers transmit the uplink through another allocated subchannel, and hence, accurate uplink synchronization is required according to the OFDMA symbol processing characteristic. When the OFDMA uplink symbol of the subscriber station is not accurately synchronized, a signal of a specific subscriber station generates interference to a signal of another subscriber station and the base station may fail to accurately detect signals.
In order to prevent the above-described situation, the subscriber station transmits a pseudo random binary sequence (PRBS) to the base station, the base station detects the same to estimate a delay distance so that each subscriber station is synchronized when the base station is synchronized, which is called a ranging method. In the ranging method, on accessing the base station, the subscriber station transmits a ranging signal for correcting a timing error.
The ranging in the OFDMA system is classified as initial ranging, hand-off ranging, bandwidth request ranging, and periodic ranging.
The initial ranging controls the subscriber station's transmission power, time, and frequency offset when the subscriber station initially accesses the base station, and the initial ranging is performed to start an initial signal access process.
The hand-off ranging is performed to control transmission power, time, and frequency offset for accessing a new base station when the subscriber station is handed off to another base station.
The bandwidth request ranging is performed to request a bandwidth for transmitting uplink traffic generated to the subscriber station.
The periodic ranging accurately measures varied power, time, and frequency offset when the subscriber station moves to change the delay distance, and the periodic ranging is performed at regular time intervals so as to correct the varied values.
The initial ranging is used for the subscriber station to be initially synchronized with the corresponding base station, and the subscriber station receives a downlink and transmits a ranging signal. In the TDD method, a TTG is provided between the downlink and the uplink, and the TTG relating to the cell radius of the base station has a value which is greater than twice the maximum propagation delay between the base station and the subscriber station, that is, greater than the round trip delay. Accordingly, the initial ranging signal is transmitted in the TTG interval.
For the above-described initial ranging, the OFDM symbol is transmitted through two symbol intervals. When the OFDM symbol reaches a transmitted signal transmission interval or a received signal transmission interval, the base station loses a valid symbol and the subscriber station transmits the OFDM symbol through two consecutive symbol intervals.
When periodic ranging and a bandwidth request ranging are performed after initial ranging is performed, the subscriber station transmits a ranging signal through an OFDM symbol so that a timing error is generated in a guard interval.
In this instance, when the subscriber station approaches the base station or leaves the base station, a downlink receiving time of the subscriber station and an uplink receiving time of the base station are varied.
FIG. 1 shows a schematic diagram for a TDD frame transmitted and received when the subscriber station moves after initial synchronization by the OFDMA system, and FIG. 2 shows a schematic diagram for generation of interference when the subscriber station approaches the base station after initial synchronization.
As shown in FIG. 1, the signal transmitted and received between the subscriber station and the base station after initial synchronization is given as (a). However, when the subscriber station approaches the base station as given in (b), a downlink receiving time by the subscriber station and an uplink receiving time by the base station are quickened. When the subscriber station leaves the base station as given in (c), the downlink receiving time by the subscriber station and the uplink receiving time by the base station are delayed.
That is, the propagation delay between the base station and the subscriber station is variable by the position movement of the subscriber station. Therefore, when the subscriber station accurately perform uplink time synchronization and the subscriber station approaches the base station after initial ranging, interference of a next symbol is generated in the interval for performing fast Fourier transform (FFT) by a base station demodulator because of decrease of propagation delay as shown in FIG. 2.
As shown in FIG. 2, in order to access the base station through an uplink, the subscriber station transmits OFDM symbols during a given time slot. In this instance, when the subscriber station transmits uplink data by using allocated subcarriers, orthogonality between the subcarriers is lost and interference is generated when a plurality of subscriber stations are not synchronized as the subscriber station approaches or leaves the base station.
In this instance, when the periodic ranging is processed quickly, it is required to measure the timing error using the ranging in the OFDM symbol guard interval, and hence, the interference generated in the symbol is not solved. Also, when the subscriber station transmits the periodic ranging signal through two OFDM symbol intervals in a like manner of the initial ranging, the problem of degrading the system capacity and performance may be generated according to the period of the periodic ranging.